1. The Field of the Invention
The present invention generally relates to semiconductor laser devices. More particularly, the present invention relates to a distributed feedback laser device having a structure that improves both manufacturing yield and operating performance of the laser device.
2. The Related Technology
Semiconductor lasers are currently used in a variety of technologies and applications, including optical communications networks. One type of semiconductor laser is the distributed feedback (DFB) laser. The DFB laser produces a stream of coherent, monochromatic light by stimulating photon emission from a solid state material. DFB lasers are commonly used in optical transmitters, which are responsible for modulating electrical signals into optical signals for transmission via an optical communication network.
Generally, a DFB laser includes a positively or negatively doped bottom layer or substrate, and a top layer that is oppositely doped with respect to the bottom layer. An active region, bounded by confinement regions, is included at the junction of the two layers. These structures together form the laser body. A coherent stream of light that is produced in the active region of the DFB laser can be emitted through either longitudinal end, or facet, of the laser body. One facet is typically coated with a high reflective material that redirects photons produced in the active region toward the other facet in order to maximize the emission of coherent light from that facet end.
A grating is included in either the top or bottom layer to assist in producing a coherent photon beam. For example, the grating is typically produced in the top layer of the DFB laser body by depositing a first p-doped top layer having a first index of refraction atop the active region, then etching evenly spaced grooves into the first top layer to form a tooth and gap cross sectional grating structure along the length of the grating. A second p-doped top layer having a second index of refraction is deposited atop the first top layer such that it covers and fills in the grating structure. During operation of the DFB laser, the tooth and gap structure of the grating, which is overlapped by optical field patterns created in the active region, provides reflective surfaces that couple both forward and backward propagating coherent light waves that are produced in the active region of the laser. Thus, the grating provides feedback, thereby allowing the active region to support coherent light wave oscillation. This feedback occurs along the length of the grating, hence the name of distributed feedback laser. After reflection is complete, the amplified light waves are then output via the output end facet as a coherent light signal. DFB lasers are typically known as single mode devices as they produce light signals at one of several distinct wavelengths, such as 1,310 nm or 1,550 nm. Such light signals are appropriate for use in transmitting information over great distances via an optical communications network.
DFB lasers as described above are typically mass produced on semiconductor wafers. Many DFB laser devices can be formed on a single wafer. After fabrication, the DFB lasers are separated from one another by a cleaving process, which cuts each device from the wafer. This cleaving process creates each end facet of the DFB device body. Unfortunately, limitations inherent in the cleaving process do not allow the laser device to be cut such that a precisely desired distance is established between the end facet and the nearest adjacent grating tooth.
The inherent variability of the distance between the end facet and the adjacent grating tooth created as a result of cleaving can cause several problems. First, the end facet, especially an end facet that is coated with a high reflective coating, may be disposed adjacent the nearest grating tooth such that the laser during operation will exhibit poor sidemode suppression, which in turn results in undesired optical frequencies being amplified within the laser device. These undesired optical frequencies can spoil the monochromatic nature of the DFB laser output and result in reduced performance for the apparatus in which the laser device is disposed.
Other problems that can arise from the arbitrary cleaving process include an increased incidence of chirp and low power output from the DFB laser device. Chirp, or the drifting of the optical output wavelength over time, is magnified by improper distances between the grating and the high-reflective end facet caused by the cleaving process. Similarly, low power output is evidence of less-than-ideal cleaving of the DFB laser device.
If one or more of the above-described problems is detected in a particular DFB laser device after manufacture and testing, it often must be discarded, thereby lowering the yield of acceptable DFB laser devices that are produced from a wafer. In some cases, the percentage of rejected devices suffering from any of the above problems can exceed 50% per wafer.
Attempts to mitigate the effects of low precision cleaving have involved the addition of one or more quarter phase shifts in the grating. However, the typical DFB grating has a continuous pattern over the entire wafer. This continuous pattern allows for the lithography to be simple. Yet, the installation of one or more quarter phase shifts requires the use of a special lithography apparatus. Additionally, special techniques are required in order to add such phase shifts. These special requirements necessarily increase the cost of production of each DFB device.
In light of the above, it would be desirable to enable the production of DFB laser devices where the yield per wafer is substantially increased. Further, a need exists for the DFB laser to exhibit good sidemode suppression while limiting chirp and output power loss. Moreover, such a solution should be simply implemented, thereby limiting production cost increases.